support decorator and context manager

[cgarciae]

Now also works as a context manager:

with stop_iter() as it:
  for n in integers():
    print(n)
    if it.interrupt:
      break

And as a decorator:

@(it := stop_iter())
def print_integers():
  for n in integers():
    print(n)
    if it.interrupt:
      break

print_integers()